A base sheet for dry processing type image-transferring materials which have an image printed by a thermal image-transferring process is described in U.S. Pat. No. 4,870,427. In the patent, a polyethylene film, a polypropylene film and a fluorine-based resin film having a smooth surface and exhibiting a contact angle with water of at least 95.degree., and paper, metal foil or plastic films having a release coating of a silicone resin are used as a base sheet.
In order to thermally print an image onto the base sheet having a contact angle with water of at least 95.degree. and particularly not less than 105.degree. it is necessary to reduce the surface tension of the ink to wet the sheet and it is further necessary to increase adhesion between the ink and the sheet more than the cohesive force of the ink and the adhesion between the ink and the sheet of an ink releasing material, such as polyethylene terephthalate film, on which the ink is carried. To achieve this result, the ink temperature must be increased when the image is thermally printed, requiring high energy to be applied to a thermal image-transferring device, which is disadvantageous from the standpoints of durability of a thermal head and load on a power supply.
Further, an image thermally printed on the base sheet having poor wettability is easily retransferred with slight pressure due to weak adhesion to the base sheet. So, a portion of the image which is desired to be left on the base sheet is unintentionally retransferred, causing stains on an image-receiving material. Such easy transfer is also troublesome in handling of the base sheet.
Furthermore, since the base sheet has an extremely small static friction coefficient, the sheet is not easily fixed during retransfer of the thermally printed image from the sheet to an image-receiving material. Therefore, the image is retransferred onto an undesired portion of the image receiving material or distorted on the image receiving material.
Japanese Laid-Open Patent Publication No. 63-246298 discloses formation of a sticky layer apart from a thermally printed image on a base sheet so as to prevent the sheet from moving during retransfer of the image with pressure.
However, the formation of a sticky layer on a certain portion of the base sheet necessitates a specific means in production and an exclusive device therefore, requiring large costs. Further, the sticky layer has to be covered with a separable sheet before use, i.e., before the image retransfer step, which requires additional means and costs. Moreover, in the case of a base sheet having a release coating of a silicone resin, the base sheet is coated twice for the release coating and the sticky layer. Regardless of the coating order of the two, the later coating may have chance to be affected by the previous coating.
That is, when a silicone resin is first coated on the sheet, the subsequent coating of a sticky composition is repelled. When the sticky composition is first coated, on the other hand, the silicone resin is coated only with difficulty because of the stickiness of the previous coating. If the sticky layer is covered with a separable sheet, then the thickness of the resulting sheet partially increases so that the silicone resin cannot easily be coated. Even if the above processing works properly to coat the silicone resin, since the sticky layer is provided only at a certain portion of the sheet and not around images thermally printed on the sheet, the sheet still moves during the image-retransfer step resulting in formation of imperfect images on the image-receiving material.
Surface treating agents such as, for example, release coating used in conventional image-retransfer sheets, are to improve the property of retransferring an ink image from the base sheets, and they are not transferred to an image-receiving material with the ink image in all cases. Therefore, the image-retransfer sheets necessarily have poor wettability so as to reduce adhesion between the sheet and the ink image. Because of this feature of such image-retransfer sheets, however, the sheets suffer from various problems such as reduced capability in receiving an ink image thermally printed or transferred from an ink ribbon, increased energy needed for thermally transferring an ink image, and poor resistance to friction of an ink image thermally transferred.
In order to solve the above-mentioned problems, Japanese Laid-Open Publication No. 2-81684 and No. 2-88294 disclose a retransfer sheet by which an ink image is retransferred to a image-receiving material together with a layer of a surface treating agent. The retransfer sheet is comprised of a substrate and a layer of a surface treating agent coated on the substrate.
However, when an image on the image-retransfer sheet is retransferred to the image-receiving material with a layer of a surface treating agent, there are problems mentioned below. When the image-retransfer sheet is put on the image-receiving material and pressure is applied onto the image to be retransferred on the image-retransfer sheet by an operator, an undesirable surface treating agent around the image may be retransferred to the image-receiving sheet. This problem is not so serious if the surface treating agent is colorless.
However, if coloring agents are added to the surface treating agent and portions of such surface treating agents around the image are retransferred to the image-receiving material, lines of the image retransferred to the image-receiving material with pressure become too bold or the retransferred image may be distorted. As a result, it becomes difficult to recognize the form of the image.